The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium with improved mechanical characteristics and reduced noise.
In accordance with the recent trend of smaller device sizes and increased amounts of information, much smaller sizes and higher recording density are required for magnetic memory devices together with increased cost reduction. To meet the requirements, disks using resins as substrate materials are now under development.
To increase the recording density without any increase in device sizes, it is necessary to increase area recording density. To that effect, it is necessary to make the recording wavelength smaller (i.e. to increase a linear recording density) and make the recording track width smaller (i.e. to increase a track density), in other words, to make a recorded pattern smaller.
In that case, reduction in signal/noise ratio (SNR) is a problem. As the recording wavelength becomes smaller to approach the size of the gap length of a magnetic head, the signal output is decreased due to the gap loss. As the track width becomes smaller, the signal output is inevitably decreased in direct proportion to the decreasing track width. Decrease in the signal output in the shortest recording wavelength region can be prevented by making the gap length smaller and also by making the spacing between the recording medium and the magnetic head smaller, but in that case it is necessary to lower the simultaneously increasing noise level of the recording medium, thereby increasing the SNR. To make the spacing between the recording medium and the magnetic head smaller, it is also necessary to make the surface of the recording medium as flat as possible.
In case of using a CoCr alloy thin film as a recording magnetic layer, the medium noise has been lowered by a process procedure of forming a CoCr alloy thin film on a substrate by vacuum film forming such as vacuum vapor deposition, etc. while heating the substrate to 250.degree. C. or higher, depositing non-magnetic materials composed mainly of Cr between grains of Co alloy magnetic particles, thereby magnetically isolating the Co alloy magnetic particles and making the particle sizes of the magnetic particles smaller. However, the process procedure cannot be applied to magnetic disks using ordinary resins having a softening temperature of 100.degree.-250.degree. C. as substrates.
Another reported process for lowering the noise level without substrate heating, comprises dispersing non-magnetic materials such as SiOx, etc. in the Co alloy thin film recording layer, thereby isolating the magnetic particles, has been reported, but the process requires an RF magnetron sputtering procedure, because non-electroconductive oxides, nitrides, etc. must be sputtered. Generally, the RF magnetron sputtering procedure is liable to make a broader plasma extension than DC magnetron sputtering procedure, and thus the substrate temperature is liable to elevate. In the case of magnetic disks, film formation must be carried out on both sides of a disk and thus the disk must be supported at the outermost periphery of the disk, resulting in such problems as poor heat runaway during the sputtering and substrate deformation by heat, when the magnetic disk is based on a substrate using resin having a low softening temperature.
Furthermore, there are still the following problems besides the above-mentioned problem [i.e. (1) to lower the noise level]. (2) Since the resin has a water absorption of at least about 1% in contrast to Al and glass with substantially zero water absorption, corrosion of the recording film due to the moisture contained in the substrate is more liable to occur than in case of Al or glass substrate. (3) Since the resin is considerably soft, as compared with Al and glass, the resin substrate is liable to be damaged or scored, when a magnetic head slides on the resin substrate medium or collides with the resin substrate. (4) Since there is a large difference in the coefficient of thermal expansion between the resin and Cr or Co alloy constituting the recording film, cracks are liable to occur on the resin substrate, when a metallic thin film of Cr or Co alloy is formed thereon.
To solve the above-mentioned problems (2), (3) and (4), the following processes have been proposed: processes for forming a metallic film of Al, Ti, Cu, Zn, Ag, In or the like as a buffer layer between the resin substrate and the recording film (JP-A 4-143920 and JP-A 5-159266); processes for providing a ceramic film such as an aluminum oxide film (JP-A 64-42022), an SiN film (JP-A 64-42023), a SiC film (JP-A 64-42024), an alternately laminated film of soft SiC and soft SiC (JP-A 2-96918), a laminated film of SiN having a compression stress and SiN having a tensile stress (JP-A 2-96919), an alternately laminated film of soft BN and hard BN (JP-A 2-96920), an alternately laminated film of non-magnetic metallic layers and ceramic layers (JP-A 2-96921), a B film (JP-A 2-62713), etc.; and a process for forming a single layer or a plurality of layers of Ti, Ti alloy, or Ti compound (oxide, nitride and sulfide) on an aluminum oxide or silicon oxide-formed resin substrate or directly on a resin substrate (JP-A 61-167117).
In case of using a metallic film as a buffer layer, since there is no conformity in the crystal structure and lattice constant among the metallic buffer layer, the underlayer for the magnetic recording layer and the magnetic layer, crystal growth of magnetic layer is inhibited, resulting in deterioration of magnetic properties, and any improvement of electromagnetic conversion characteristics cannot be expected. Furthermore, since the metal is softer than the ceramic, there is the problem that the recording film is liable to be damaged at CSS (Contact-Start-Stop).
In case of using a ceramic buffer layer (JP-A 64-42022, 42023, 42024, etc.) on the other hand, the above-mentioned problem that the recording film is liable to be damaged relative to the metallic film buffer layer can be overcome, because the ceramic has a considerably higher hardness than that of the metal. However, the ceramic layer has a very uniform surface and thus the recording layer to be grown thereon will have a high uniformity, resulting in insufficient isolation of magnetic particles during the low temperature film formation, thereby increasing noise components.
In the process for alternately laminating non-magnetic metallic thin films and ceramic thin films as a plurality of layers, respectively, (JP-A 2-96921), the noise components are increased, as in case of using the above-mentioned ceramic layer only as a buffer layer, when the outermost surface buffer layer is a ceramic layer. Results of the present inventors' study showed that even in case that the outermost surface layer was a non-magnetic metallic layer, cracks developed and no satisfactory results were obtained when conditions for forming a non-magnetic underlayer was made higher than the Ar gas pressure conditions as disclosed in JP-A 2-96921 to promote physical isolation of recording film layer, thereby increasing SNR. Furthermore, the disclosure of JP-A 2-96921 makes it imperative to laminate substantially at least 5 buffer layers, and in case of production in an in-line type or single piece type sputtering apparatus, a corresponding number of sputtering chambers to the number of laminated layers will be required, resulting in a higher apparatus cost.
When an oxide ceramic buffer layer is formed as the lowest layer, and even if a layer of Ti, Ti alloy or Ti compound is formed thereon, oxygen atoms of the oxide ceramic buffer layer migrate through crystal grain boundaries of Ti, Ti alloy or Ti compound layer into the upper layer, thereby oxidizing the underlayer and the recording magnetic layer, resulting in deterioration of the magnetic characteristics, when exposed to a high temperature or high humidity conditions, since the Ti, Ti alloy or Ti compound layer is crystalline.
As described above, in forming a buffer layer between the resin substrate and the underlayer of a magnetic disk using the resin substrate, the metallic buffer layer has such disadvantages as deterioration of the magnetic characteristics and deterioration at CSS, and the ceramic buffer layer has such disadvantages as deterioration of SNR due to the increased noise level and deterioration of mechanical characteristics. Furthermore, in case of alternately laminating non-magnetic metallic thin films and ceramic thin films as a plurality of layers, respectively, (JP-A 2-96921), cracks develops and no satisfactory results can be obtained, when the Ar gas pressure is increased to obtain an increased SNR during the formation of non-magnetic underlayer.